See title: the target macro TARGET_VECTOR_ALIGNMENT_REACHABLE isn't documented. It'd be expected so in in gcc/doc/tm.texi.
Confirmed.
Adding Dorit Nuzman and Devang Patel to the CC list since they are listed as the authors of the patch that created this macro.
It was renamed to TARGET_VECTORIZE_VECTOR_ALIGNMENT_REACHABLE and documented r0-98468.